Lithium Ion Batteries (“LIB”) are a class of rechargeable batteries in which lithium ions move from a negatively-charged electrode to a positively-charged electrode during discharge and vice-versa during recharge. The rechargeability of LIBs makes them particularly useful in consumer electronics and military applications (such as electric and aerospace vehicles). Yet, conventional LIBs have some inherent safety hazards, including, probably most significantly, pressurized, liquid-based, non-aqueous, flammable electrolytes. Exemplary electrolytes of this class include lithium salts (such as, LiPF6, LiBF4, or LiClO4) in an organic solvent (such as, ethylene carbonate, dimethyl carbonate, or diethyl carbonate), having ion conductivities, at room temperature (about 20° C.) of about 10 mS/cm. The conductivities increase (30% to 40%) at temperatures of about 40° C. and decrease at 0 or below.
One solution to the flammability hazard has been the use of solid-state electrolytes (“SSE”). In some instances, conventional SSEs have specific ionic conductivities on the order of 10−3 S/cm (at room temperature), which can make the use of SSEs comparable to their liquid-based counterparts. In fact, when the lithium transfer number is 1, concentration gradients at high discharge rates can be avoided with SSEs. Some conventional SSEs presently in use in LIBs include: (1) highly lithium ion conducting glass-ceramic solid-SSEs based on Li2S—P2S5 and having an energy of activation (“Ea”) of 12 kJ/mol; (2) thio-LISICON-lithium superionic conductor SSEs (Li3.25S—Ge0.25—P0.75S4) having an Ea of about 20-45 kJ/mol but as a function of heat treatment temperature; and (3) Ohara glass ceramic, having an Ea of about 35 kJ/mol. Use of the Li2S—P2S5-based SSEs requires the use of an Argon-filled dry box. Despite the fast lithium conduction characteristic of these conventional SSEs, cell component integration associated with high-impedances at the glass-ceramic solid-state electrolyte/electrode interface can impede the use of these materials. Some have used intermediate layers, such as Lithium Phosphorus Oxynitride (“LIPON”) or other polymer-based electrolytes, to facilitate a connection between the glass-ceramic electrolyte and electrode. However, LIPON has a low specific ionic conductivity (10−6 S/cm) and a high Ea of 53 kJ/mol at room temperature. Moreover, sputter deposition is required for integrating LIPON into an electrochemical cell, which makes cost and fabrication of large area, pin-hole free electrolytes significant issues.
Thus, there remains a need for materials suitable for use in LIB cells having the operability of liquid-based electrolytes with the stability and safety associated with SSEs.